1. Field of the Invention
The present invention relates to a magnetic recording medium substrate and a hard disk drive.
2. Description of the Related Art
In recent years, with respect to a magnetic recording medium used for a hard disk drive, recording density has significantly increased. In particular, surface recording density has further drastically increased since introduction of MR (magneto resistive) head and PRML (Partial Response Maximum Likelihood) technology.
Furthermore, due to development of the Internet and expanded utilization of “Big Data” in recent years, the data accumulation amount at data centers has been continuously increasing, and thus, due to spatial limitations at the data centers, increases in memory capacity per unit volume are required. In other words, in order to increase memory capacity per standardized hard disk drives, attempts have been made for increasing the number of magnetic recording media stored inside drive cases in addition to attempts to increase memory capacity per magnetic recording medium.
As a magnetic recording medium substrate, an aluminum alloy substrate and a glass substrate are mainly used. In the substrates described above, the aluminum alloy substrate has a toughness higher than the glass substrate, and is easier to manufacture, and thus, the aluminum alloy substrate is used for a magnetic recording medium with a relatively large diameter. The thickness of an aluminum alloy substrate, which is used for a magnetic recording medium of a typical 3.5-inch standardized hard disk drive, is 1.27 mm, and up to five (5) aluminum alloy substrates are used inside the drive case.
In order to increase the number of magnetic recording media stored inside a drive case, attempts have been made for making thinner the thickness of the substrate used for the magnetic recording medium. However, in the case where the thickness of the substrate is made thinner, compared with a glass substrate, the aluminum alloy substrate tends to generate “fluttering”. F is a phenomenon generated in a magnetic recording medium in the case where the magnetic recording medium is rotated at a high speed. When the fluttering is generated, it becomes difficult to perform stable reading in the hard disk drive.
In order to reduce the fluttering, in the glass substrate, for example, it is known that a material with high Young's modulus is used as a material of a magnetic recording medium substrate (for example, refer to Patent Document 1).
Further, it is known that the fluttering of the substrate is reduced by filling helium gas inside the drive case of a 3.5-inch standardized hard disk drive, and thus, the thickness of the aluminum alloy substrate is made thinner and six or more aluminum alloy substrates are stored inside the drive case.
In general, an aluminum alloy substrate is manufactured according to the following process. First, punching in a donut shape is performed on an aluminum alloy plate with a thickness of approximately 2 mm or less, and a substrate with a desired size is obtained. Next, after performing inner and outer diameter chamfering and data surface turning on the substrate obtained by the punching, in order to reduce surface roughness and undulation after the lathe manufacturing, grinding is performed by using a grindstone. Afterwards, in order to provide surface hardness and reduce surface defect, NiP plating is performed on the substrate surface. Next, polishing processing is performed for both surfaces (data surfaces) of the substrate on which NiP plating films have been formed. Substrates for magnetic recording mediums are mass production products, and high cost-performance is required for magnetic recording mediums. Therefore, high machinability and inexpensiveness are required for the aluminum alloy used for the substrates.
Patent Document 2 discloses a material as an aluminum alloy with superior machinability, capable of reducing the wear, chipping, etc., of cutting tools, and with further improved Alumite processability. The material includes Mg: 0.3 to 6 mass %, Si: 0.3 to 10 mass %, Zn: 0.05 to 1 mass %, and Sr: 0.001 to 0.3 mass %, with the balance being Al and impurities.